1. Technical Field
Example embodiments of the present invention relate to a semiconductor memory device, for example, to a phase change memory device.
2. Description of Related Art
A semiconductor memory device is a memory device that stores data and from which stored data may be read. The semiconductor memory device may be divided into a volatile memory device and a nonvolatile memory device.
The volatile memory device is a memory device in which retained data is erased when a power supply is cut off. Some examples of the volatile memory device are SRAM, DRAM and SDRAM. The nonvolatile memory device is a memory device in which retained data is not erased even when power supply is cut off. Some examples of the nonvolatile memory device are ROM, PROM, EPROM, EEPROM, flash memory device, PRAM, MRAM, RRAM, and FRAM.
A phase change memory (PRAM) is a memory device that stores data using a material of which a resistance varies with temperature, for example, chalcogenide. When a temperature higher than the melting point of chalcogenide is applied to the chalcogenide for a relatively shorter period of time, the chalcogenide becomes an amorphous state. When a temperature lower than the melting point of chalcogenide is applied to the chalcogenide for a longer period of time, the chalcogenide becomes a crystalline state. A resistance of the chalcogenide in the crystalline state is lower than a resistance of the chalcogenide in the amorphous state. Thus, the phase change memory device may store data by changing chalcogenide into a crystalline state or an amorphous state.